1. Field of the Invention
The present disclosure relates to a technique of controlling motors for driving rollers to convey a sheet material to be used in an image forming apparatus.
2. Description of the Related Art
A small and inexpensive stepping motor is often used as a drive source for a sheet feeding/conveying system in an image forming apparatus. The stepping motor rotates by a unit step angle in response to a 1-pulse input. This clear relationship between the input and output enables the use of open-loop control to simplify the system including a control mechanism. While the stepping motor can contribute to achieving a compact and inexpensive structure, the stepping motor frequently causes a step-out phenomenon in which the rotation of a rotor cannot be synchronized with the input of a pulse signal. The step-out phenomenon occurs, for example, in an overload state to the pulse rate of the pulse output to the stepping motor from a drive circuit.
An image forming apparatus that performs multifarious types of image formation needs to support various kinds of sheets, such as plain paper and thick paper. Therefore, the required torque of the stepping motor varies significantly depending on the kind of sheets in use. With regard to the torque for causing a sheet to enter between conveying rollers made of a sponge material and disposed along a sheet conveying path, for example, the torque for thick paper (200 g/cm) becomes twice as high as the torque for plain paper (80 g/cm) in some cases. The torque of the stepping motor is determined by the value of the drive current. Therefore, the selection of the stepping motor and the setting of the drive current that determines the torque are determined on the assumption of using thick paper which faces severer conditions.
However, plain paper is frequently used in general offices or the like. Therefore, constant setting of the drive current value for thick paper causes more than necessary power consumption and noise generation. Therefore, a technique to solve this problem is proposed (see U.S. Pat. No. 7,547,016). The apparatus described in U.S. Pat. No. 7,547,016 uses a feedback stepping motor incorporating a sensor which detects the position of the rotor, as an upstream (sheet feeding system) motor. The feedback stepping motor monitors information on the rotational speed and the amount of rotation during rotation by means of the sensor, and performs closed-mode control immediately when a step-out phenomenon is likely to occur. The apparatus described in U.S. Pat. No. 7,547,016 determines the value of the current to a downstream (conveying system) stepping motor based on the result of detection from the sensor.
Recently, there is a proposal of a technique of optimizing the drive current based on the relationship between the maximum output torque of a stepping motor and the level of the drive current (see US 2011/0229235). The technique disclosed in US 2011/0229235 calculates an estimated load torque at the time of conveying a first sheet. The estimated load torque is used to determine a current value corresponding to the size of the target load torque at the time of conveying second and subsequent sheets.
The technique disclosed in U.S. Pat. No. 7,547,016 is effective in preventing a step-out phenomenon. When a plurality of sheet cassettes are mounted for the respective kinds of sheets, however, feedback stepping motors are needed for the respective sheet cassettes. This complicates the control process. Further, the feedback stepping motors are very expensive and undesirably lead to an increase in the cost for the image forming apparatus.
The technique disclosed in US 2011/0229235 independently controls a plurality of stepping motors, thus complicating the control circuit when a plurality of stepping motors are used.